MOLYBDENITE
Molybdenite – MoS2 – is the most abundant molybdenum mineral and the main molybdenum ore mineral. It is the only rhenium ore, as it always contains Re in levels from ppm to 1-2%.
Jordisite is still amorphous MoS2, found in semi-metallic masses in some deposits. Wulfenite – Pb(MoO4) – is formed from jordisite. Jordisite alters very easily, even in collections, to ilsemannite – Mo3O8.nH2O. “Muchuanite” is a mixture of molybdenite and jordisite.
Due to its very low hardness, the trace of molybdenite can be obtained by scratching the mineral on a sheet of paper. The mineral feels unctuous (greasy) and handling it dirties the fingers. It is dimorphic with jordisite. There are two polytypes of molybdenite, which cannot be distinguished under a microscope: molybdenite-2H is common and molybdenite-3R is rare; they can occur together and/or intergrown.
Molybdenite is very similar to graphite macroscopically; the distinction can be very difficult or impossible. Graphite has a darker, silvery-black color (sometimes resembling biotite) and often has a somewhat more bluish hue. The density of molybdenite is greater than that of graphite, but this property is generally difficult to assess. Molybdenite alters powellite; this substitution may only be partial. Powellite is intensely fluorescent in white-blue colors and therefore easy to recognize.
“Castaingite” was a term attributed to CuMo2S5, but it has been discredited. There are several unnamed Cu-Mo sulfides, one Pb-Mo sulfide, and another K-Mo sulfide, showing the complexity of possible combinations.
1. Characteristics
Crystal system: Hexagonal holohedral.
Color: Black to silvery lead gray, bluish-black, a bluish tone is quite characteristic.
Habit: Lamellar, scaly, cleavable masses, scattered grains, foliated. Rarely as tabular hexagonal crystals.
Cleavage: {0001} perfect. Trigonal markings in {0001}, parallel to the line {10-11}.
Tenacity: Flexible, non-elastic sheets.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 1 – 1.5
Parting: No.
Streak: Bluish gray.
Lustre: Metallic white-bluish.
Diaphaneity: Opaque, translucent in very fine flakes.
Density (g/cm³): 4.7 – 4.8
2. Geology and Deposits
It occurs primarily in high-temperature hydrothermal veins. Important deposits are found in porphyry-type deposits, with or without widespread associated copper mineralization.
It also occurs in contact metamorphic deposits with limestones (skarns). Also in associated granites, syenites, aplites, and pegmatites and, rarely, in meteorites.
3. Mineral Associations
Molybdenite is a sulfide that can be found in a variety of geological environments.
In porphyry copper deposits, it occurs with chalcopyrite, chalcocite, bornite, cubanite, tennantite, sphalerite, and a number of secondary copper minerals.
In quartz-rich pegmatites and greisen, it is associated with cassiterite, wolframite, bismuthinite, and other sulfides, including several bismuth-containing sulfides.
In skarns, it can be found with calcite, dolomite, and others; skarns can be mineralized with Cu, Cu-Au, or W, with all the characteristic ore minerals of each element.
It is mainly a commercial source in high-temperature hydrothermal veins, occurring with scheelite, wolframite, topaz, rutile, zinkenite, and fluorite.
In stockwork deposits, in veins with Sn-W and uraninite ore minerals.
In magmatic sulfides of basic rocks, it is associated with pyrite, pyrrhotite, and pentlandite.
It occurs occasionally in plutonic rocks such as syenites.
It can also be found in high-grade metamorphic rocks, such as gneisses.
It can form in volcanic fumaroles.
In sedimentary rocks, its formation as a biogenic mineral is possible, thus explaining the analytically found Mo values.
Rolled molybdenite grains can occur in ancient (Precambrian) and modern alluvial gold deposits.
4. Transmitted Light Microscopy
Not applicable, as molybdenite is opaque. Extremely thin lamellae of molybdenite are translucent, garlic green in color.
5. Reflected Light Microscopy
Sample preparation: Polishing molybdenite is difficult due to its low hardness (lower than graphite). The mineral tends to smudge, resulting in many polishing grooves at best. Polishing is especially difficult in aggregates of large grains and in the presence of quartz.
Polishing is easier when it occurs as aggregates of fine fibers associated with calcite and other soft minerals. The hardness upon polishing is greater than expected; similar to that of galena and chalcopyrite, but lower than the hardness of calcite and graphite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray to bluish-gray, also creamy-yellowish gray. The color depends on the section of the mineral; it can even be pure white.
Pleochroism: Very strong, ranging between bluish-white and cream. This pleochroism, which is one of the strongest known, can appear as distinct shades of gray and may have a defined blue tone.
Reflectivity: 20,51 – 44,27%
Bireflectance: Strong.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Extremely strong anisotropy in shades of brown, blue, and turquoise. The bluish tone is very characteristic and diagnostic. But with the nicols exactly crossed, the colors are not expressive – a white with a pinkish tone. It is necessary to rotate one of the nicols minimally (1-2º) to observe the deep blue and other colors.
Internal reflections: No.
May be confused with: Graphite exhibits similar behavior and habits, but in XPL it is darker and displays yellowish colors instead of the bluish hues of molybdenite. Valeriite and makinawite have higher reflectivity and more yellowish tones. Theoretically, it is possible to confuse it with tetradymite, nagyagite, and some rare minerals.
General Characteristics:
Grain shape: its occurrence in lamellae, often deformed, folded and stacked, is very characteristic. In the deformed lamellae, extinction is incomplete and wavy. The grains may have a barrel shape.
Colloform textures are possible.
Cleavage parallel to {0001} is almost always visible if polishing was done well.
Grain size is highly variable. Pegmatites are known to have plates with a diameter of 15 cm; in masses formed from gels, the grains are at the limit of observability under a microscope. In most occurrences, the granulation is relatively coarse. In masses consisting only of molybdenite, the intergranular contacts are straight, with very little toothing. Tuff or feathery aggregates are rare.
Mammillary to botryoidal masses (as in goethite), formed by globules of varying sizes with a few millimeters in diameter, develop a very distinct concentric texture that is highlighted by polishing due to the greater hardness of the larger diameter globules.
Parallel dislocations (crumpling) may be very well developed, producing a texture that resembles twinning.
Cataclasis is not expected in such an easily deformable mineral, but compressions very often result in delaminations (an “open book” structure).
Deformations are extremely common and always present in coarse-grained masses. Frequently, widespread deformations are easily observable even in uncrossed nicoles. In some cases, extremely deformed molybdenite lamellae are situated amidst grains of other minerals that apparently have not undergone any type of deformation, which can be explained by the recrystallization of these other minerals after tectonic events; molybdenite preserved the features generated by tectonism while other minerals, such as chalcopyrite, recrystallized.
Polysynthetic twins of primary origin are indicated by variations in polarization effects in sections parallel to “c”.
Substitutions 1: molybdenite can be replaced by powellite, arsenopyrite, and chalcopyrite.
Substitutions 2: molybdenite replaces pyrite, chalcopyrite, arsenopyrite, and magnetite.
Alteration to molybdite (MoO3) is common and accompanied by partial pseudomorphization. This oxidation readily follows the planes opened by delaminations.
Inclusions in molybdenite can be native bismuth, bismuthinite, ikunolite, joseite, galena, and gold.
Inclusions of molybdenite occur in pyrite, chalcopyrite, arsenopyrite, bismuthini, and tetrahedrite; they can be interstitial in cassiterite and wolframite; and small spots occur in scheelite.
